1. Field of the Invention
The present invention relates to an image formation apparatus such as copiers and printers, and a development method using such image formation apparatus.
2. Description of the Background Art
A conventional image formation apparatus will be first described.
FIG. 12 is a schematic diagram of a conventional image formation apparatus based on the electrophotographic system. Referring to FIG. 12, the image formation apparatus includes a photoreceptor 1, a charger device 2, an exposure device 3, a developing device 4, a transfer device 6, a cleaner 7, and an optical discharger lamp 8. Photoreceptor 1 is arranged at substantially the center region of the image formation apparatus. Around photoreceptor 1 are disposed charger device 2, exposure device 3, developing device 4, transfer device 6, cleaner 7 and optical discharger lamp 8, sequentially in the direction of rotation of photoreceptor 1.
Exposure device 3 includes a semiconductor laser 301, a polygon mirror 302 for scanning a laser beam, and a lens system 303 to direct the laser beam in a desired shape and scanning speed onto photoreceptor 1 to form an image.
In operation, photoreceptor 1 has its surface charged to a predetermined potential level by charger device 2. Then, a latent image potential corresponding to image information is formed on photoreceptor 1. The electrostatic latent image formed on photoreceptor 1 is conveyed by the rotation of photoreceptor 1 to a development region that faces developing device 4.
In this development region, a development roller is disposed facing photoreceptor 1. The development roller carries a developer precharged to a desired value and having the layer thickness adjusted (referred to as xe2x80x9ctonerxe2x80x9d hereinafter) at its surface. The toner is transferred onto photoreceptor 1 corresponding to the latent image pattern to render the image visible. Following visualization of the latent image potential on photoreceptor 1 by the toner, the toner image is conveyed to a transfer region located at transfer device 6 through the rotation of photoreceptor 1.
A transfer sheet P fed by a sheet feed device not shown is delivered to the transfer region to be synchronously brought into contact with the toner image on photoreceptor 1. A voltage of a polarity of either state corresponding to transferring the toner on photoreceptor 1 to transfer sheet P is applied to transfer device 6, whereby the toner image on photoreceptor 1 is transferred onto transfer sheet P. Transfer sheet P with the toner image is then delivered to have the image fused and fixed on transfer sheet P by a thermal fixing device (not shown). The untransferred toner remaining on photoreceptor 1 after passage of the transfer region is removed from photoreceptor 1 by cleaner 7. A refresh operation of potential is conducted by optical discharger lamp 8 to erase the residual charge of photoreceptor 1. Then, control returns to the initial process.
In the above-described electrophotographic image formation apparatus, characters and the like are binary-recorded by means of the presence/absence of dots based on the toner. In the case of a photographic image or the like, the halftone is expressed by pixels formed of a plurality of binary-recorded dots. If the number of dots in one pixel is increased in order to obtain more gray scale levels that can be represented, the pixel size will become larger. As a result, the resolution defined by the number of pixels is reduced.
To address this issue, various approaches have been employed to obtain more gray scale levels that can be expressed with one pixel without altering the pixel size. For example, the light-on time of the laser beam in the formation of a latent image of one dot is altered to change the size of one dot, or the intensity of the laser beam is altered to change the density of one dot. The technique related to pulse-width modification by altering the light-on time of the laser beam is disclosed in, for example, Japanese Patent Laying-Open No. 3-4244. This publication discloses that the controllability and stability of the gray scale can be improved at the image area of low image density by setting the spot diameter of the laser beam to not more than 0.7 times the dot pitch (63.5 xcexcm for 400 dpi) to increase the contrast of the latent image potential.
There is now a greater demand for higher resolution in the market. For example, the resolution of approximately 1200 dpi is desired so that the area of slanted lines in a character or the like can be easily identified. It is also desired that one dot is formed at approximately 20 xcexcm to improve the graininess of the highlight area.
If the writing pitch with the laser beam is reduced in accordance with higher resolution, the exposure spot must also be reduced. Consider the case of an isolated dot. Even if the exposure spot of laser is made smaller than the writing pitch, or even if the energy distribution thereof is sharp, the potential distribution will become gentle due to the diffusion of charge generated in the photoreceptor after exposure by laser. In other words, it is desirable that the potential at the surface of the photoreceptor corresponding to an isolated dot shown in FIG. 13A exhibits squareness as shown by the solid line in FIG. 13B. However, the potential corresponds to a gentle curve indicated by the chain dotted line due to charge diffusion. The potential distribution corresponding to the potential at the surface of the photoreceptor becomes gentler as the dot pitch becomes smaller. The peak value of potential (=potential difference for developing) becomes lower as indicated by the chain dotted line in FIG. 13B. Thus, an isolated dot can no longer be developed in a digital manner. An isolated dot can be developed only in an analog manner.
This issue will be described with reference to FIGS. 14-17.
FIG. 14 represents the potential distribution at the toner layer face of one dot of a latent image on the photoreceptor, formed by pulse-width modification, in the case where the dot pitch and the exposure spot are relatively large. In FIG. 14, broken line t represents the potential level of development commencement corresponding to the general developing characteristics shown in FIG. 17 whereas broken line s represents the potential level of development saturation corresponding to the same general developing characteristics shown in FIG. 17.
In the case where the dot pitch and the exposure spot are both relatively large, dots in the lower gray scale level will not be developed since the development commence level t is not reached as shown in FIG. 14. In the invention of the aforementioned Japanese Patent Laying-Open No. 3-4244, the laser spot is reduced in order to solve this problem, whereby dots in the lower gray scale level can be developed stably since the development saturation level s is exceeded as shown in FIG. 15.
However, if the dot pitch is reduced to a level so as to allow realization of the resolution of approximately 1200 dpi according to the approach disclosed in Japanese Patent Laying-Open No. 3-4244, the potential distribution of the toner layer face by a latent image will be as shown in FIG. 16 even if the laser spot is reduced. Dots of the upper gray scale level will be developed in the density increasing region (the potential between development commence level t and development saturation level s), not in the density saturation region. In this density increasing region, any slight variation in the potential of the toner layer determined by the potential distribution in the toner layer, the charged amount of toner, or the toner attaching amount of a latent image will cause variation in the dot diameter. It is therefore difficult to realize stable gray scale levels particularly in a low density region.
In view of the foregoing, an object of the present invention is to provide a development method and image formation apparatus aimed to improve picture quality and stability in development by forming a small dot stably with no graininess to realize stable gray scale in a low density region, and obtaining sufficient density in a high density region, in forming an image of high resolution.
In a development method according to an aspect of the present invention conducted by an image formation apparatus including an electrostatic latent image carrier carrying an electrostatic latent image and a developer carrier carrying a developer at its surface, facing the electrostatic latent image carrier at a developing region, the increasing ratio of development density to increase of the potential difference between the electrostatic latent image carrier and the developer carrier differs between the region where the potential difference is small and the region where the potential difference is large. Development is carried out under the developing characteristics in which the increasing ratio of development density in a region where the potential difference is small is smaller than the increasing ratio of development density in a region where the potential difference is large, and the upper limit of the development density in the region where the increasing ratio of development density is small is at least 0.3.
Since the increasing ratio of development density is small in the region where the potential difference between the electrostatic latent image carrier and the developer carrier is small according to the development method of the present aspect, variation in dots can be suppressed. Furthermore, sufficient image density can be achieved since the increasing ratio of development density is large in the region where the potential difference between the electrostatic latent image carrier and the developer carrier is large.
Accordingly, by forming small dots stably with no graininess to realize a stable gray scale in the low density region, and achieve sufficient density in the high density region, the picture quality can be improved and development can be carried out stably in the formation of an image at high resolution.
The upper limit of the development density in a region where the increasing ratio of development density is small is set to at least 0.3. This is because an isolated dot can be formed stably if the upper limit is at least 0.3. The development density in the present specification refers to the optical density of an image subjected to development.
In the development method of the present aspect, development is carried out under the developing characteristics in which the upper limit of the development density in the region where the increasing ratio of development density is small is at least 0.5.
Accordingly, an isolated dot can be formed stably even if there is a great change in potential caused by environment modification.
According to another aspect of the present invention, in a development method conducted by an image formation apparatus including an electrostatic latent image carrier carrying an electrostatic latent image and a developer carrier carrying a developer at its surface, facing the electrostatic latent image carrier at a developing region, development is conducted using a developer of a small increasing ratio of development density to increase of the potential difference between the electrostatic latent image carrier and the developer carrier, followed by development using a developer of a larger increasing ratio of development density to increase of the potential difference.
In the development method of the present aspect, development is carried out using a developer of small increasing ratio of development density to increase of the potential difference between the electrostatic latent image carrier and the developer carrier, and then development is carried out using a developer of larger increasing ratio. Therefore, the increasing ratio of development density in a region where the potential difference is small can be set smaller than the increasing ratio of development density in a region where the potential difference is large. Variation in dots can be suppressed since the increasing ratio of development density is small in the region where the potential difference between the electrostatic latent image carrier and the developer carrier is small. Also, since the increasing ratio of development density is large in the region where the potential difference between the electrostatic latent image carrier and developer carrier is large, sufficient image density can be obtained.
Accordingly, by forming small dots stably with no graininess to realize a stable gray scale in the low density region, and achieve sufficient density in the high density region, the picture quality can be improved and development can be carried out stably in the formation of an image at high resolution.
According to a further aspect of the present invention, an image formation apparatus includes an electrostatic latent image carrier carrying an electrostatic latent image, and first and second developer carriers respectively carrying a developer at its surface, and facing the electrostatic latent image carrier at a developing region. The increasing ratio of development density to increase of the potential difference between the first developer carrier and the electrostatic latent image carrier is smaller than the increasing ratio of development density to increase of the potential difference between the second developer carrier and the electrostatic latent image carrier. The first developer carrier is arranged upstream of the second developer carrier in a travel direction of the electrostatic latent image carrier.
By disposing the first developer carrier upstream of the second developer carrier in the travel direction of the electrostatic latent image carrier in the image formation apparatus of the present aspect, development can be carried using a developer of a small increasing ratio of development density to increase of the potential difference between the electrostatic latent image carrier and the developer carrier, and then development can be carried out using a developer of a larger increasing ratio of development density to increase of the potential difference. Accordingly, the increasing ratio of development density in a region where the potential difference between the electrostatic latent image carrier and the developer carrier is small becomes smaller than the increasing ratio of development density in a region where the potential difference is large. Since the increasing ratio of development density is small in the region where the potential difference between the electrostatic latent image carrier and the developer carrier is small, variation in dots can be suppressed. Also, sufficient image density can be achieved since the increasing ratio of development density is large in the region where the potential difference is large.
Accordingly, by forming small dots stably with no graininess to realize a stable gray scale in the low density region, and achieve sufficient density in the high density region, the picture quality can be improved and development can be carried out stably in the formation of an image at high resolution.
In the image formation apparatus of the present aspect, the specific charge of the developer on the first developer carrier is preferably larger than the specific charge on the second developer carrier.
Accordingly, the increasing ratio of development density to increase of the potential difference between the first developer carrier and the electrostatic latent image carrier can be set small, and the increasing ratio of development density to increase of the potential difference between the second developer carrier and the electrostatic latent image carrier can be set large.
The image formation apparatus of the present aspect preferably includes a charge generation device applying desired charge to at least one of the developer on the first developer carrier and the second developer carrier by applying charge of a single polarity.
Even if the same toner is used for two developing devices, charge can be applied by the charge generation device so that the specific charge of the developer on the first developer carrier is larger than the specific charge on the second developer carrier.
In the image formation apparatus of the present aspect, the resistance value of the first developer carrier is smaller than the resistance value of the second developer carrier.
The potential of the developer on the developer carrier becomes smaller in proportion to a larger resistance of the developer carrier. By setting the resistance of the first developer carrier smaller than the resistance of the second developer carrier, the specific charge of the developer on the first developer carrier can be set larger than the specific charge of the developer on the second developer carrier even if the same toner is employed for two developing devices.
According to still another aspect of the present invention, an image formation apparatus includes an electrostatic latent image carrier carrying an electrostatic latent image, and a developer carrier carrying a developer at its surface, and facing the electrostatic latent image carrier at a developing region. A developer of a small increasing ratio of development density to increase of the potential difference between the electrostatic latent image carrier and the developer carrier and a developer of a larger increasing ratio of development density to increase of the potential difference are formed on a single developer carrier.
In the image formation apparatus of the present aspect, uniform formation of small dots and the optical density of a solid black image can both be achieved with one developing device. Accordingly, the number of structural components can be reduced, and the overall size of the apparatus can be reduced.
The image formation apparatus of the present aspect further includes a charge generation circuit applying charge of a single polarity to the developer on the developer carrier. The developer carrier includes a high resistance portion and a low resistance portion provided alternately in the travel direction of the developer carrier.
By employing a developer carrier in which a high resistance portion and a low resistance portion are provided alternately, uniform formation of small dots and the optical density of a solid black image can both be achieved with one developing device. Accordingly, the number of structural components can be reduced, and the overall size of the apparatus can be reduced.
The image formation apparatus of the present aspect further includes a charge generation device applying a charge of a single polarity to the developer on the developer carrier. The developer carrier includes a high electrostatic capacitance portion and a low electrostatic capacitance portion provided alternately in the travel direction of the developer carrier.
By using a developer carrier having the high electrostatic capacitance portion and the low electrostatic capacitance portion provided alternately, uniform formation of small dots and the optical density of a solid black image can both be achieved with one developing device. Accordingly, the number of structural components can be reduced, and the overall size of the apparatus can be reduced.
The image formation apparatus of the present aspect further includes a charge generation device applying charge of a single polarity to the developer on the developer carrier. The image formation apparatus further includes a movable support member in contact with the developer carrier at a plane opposite to the plane where the developer carrier faces the charge generation device. The support member includes two sets of electrode patterns in the travel direction of the support member. Different voltages are applied to the two sets of electrode patterns.
By applying different voltages to the two sets of electrode patterns, uniform formation of small dots and the optical density of a solid black image can both be achieved with one developing device. Accordingly, the number of structural components can be reduced, and the overall size of the apparatus can be reduced.
In the image formation apparatus of the present aspect, the developer on the developer carrier is a mixture of two sets of developers having different average grain size. The set of the developer of smaller average grain size has higher flowability than the set of the developer of larger average grain size.
Following formation of small dots using a developer of low specific charge, development is carried out using a developer of a large specific charge. Therefore, small dots can be formed stably in a low density region, and sufficient solid black density can be achieved in the high density region.
In the image formation apparatus of the present aspect, the developer on the developer carrier is a mixture of two sets of developers having different average charge. The set of the developer of the large average charge has flowability higher than that of the set of the developer of smaller average charge.
Following formation of small dots using the developer of smaller specific charge, development is carried out using a developer of a larger specific charge. Therefore, small dots can be formed stably at the low density region, and sufficient solid black density can be obtained at the high density region.
In the image formation apparatus of the present aspect, preferably a voltage having a direct-current voltage overlaid with an alternating voltage is applied to the developer carrier.
Since development can be carried out using a developer of large specific charge after small dots are formed using a developer of low specific charge, small dots can be formed stably in the low density region. In the high density region, sufficient solid black density can be achieved.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.